A kinetic study of the reactions of Group VI-B hexacarbonyls with primary amine and halide ligands was undertaken in order to determine the possible mechanisms of these reactions. As well as the expected dissociative pathway, the reactions with the primary amines were seen to proceed by a concurrent pathway which was dependent upon the ligand concentration. Since nitrogen donor ligands are expected to be poor donor ligands, the mechanism proposed was a "dissociative interchange" mechanism which should not be too dependent upon the nucleophilicity of the ligand. Comparison of the rate constants for the amines studied as well as those of the previously investigated Lewis base ligands indicated all such reactions may proceed through the same mechanism. The similarity in rate constants for the ligand-independent and ligand-dependent pathways supports this mechanism. The rate of formation of the final product was seen to be dependent upon the square of the mercuric halide concentration. Therefore, the conversion of Fe(CO)4(HgX)2 to the final product was proposed to proceed by the successive abstraction by each HgX group of two molecules of mercuric halide. These oxidative elimination reactions are related to a chemical model for the intermediate step in the reduction of dinitrogen to ammonia and their similarities and differences are discussed.

Shen, Hao. / "July 2007." / n and 0 in "4fn/d0" in title is superscript. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2007. / Includes bibliographical references (p. 207-220). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstracts in English and Chinese.

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In the reaction between [Re(CO)5Br] and 2–lithiumthienyl, X–ligand substitution was expected. Li+{C4H3S}¯did not substitute Br¯, but an intermediate negatively charged complex was obtained (non–mobile on silica gel) and it was found that the thienyl had bonded to a carbonyl ligand, producing a dirhenium acylate complex. Such complexes are the precursors to neutral Fischer carbene complexes. After alkylation with Et3OBF4, [Re2 (CO) 9C(OEt)C4H3S] (1) was obtained, instead of a monorhenium monocarbene complex. Greater yields of 1 could be obtained, from reactions with [Re2(CO) 10] instead of [Re(CO) 5Br]. [Re2 (CO) 10] reacted with 5–lithium–2,2'–bithienyl and 2–lithium–3,6– dimethylthieno[3,2–b]thienyl and was then alkylated with Et3OBF4. The reactions proceeded smoothly and [Re2 (CO) 9C(OEt)C8H5S2] (2) and [Re2 (CO) 9C(OEt)C8H7S2] (3) were obtained. The substrates thiophene, 2,2'–bithiophene and 3,6–dimethylthieno[3,2–b]thiophene, can all be doubly lithiated under appropriate reaction conditions. These lithiated species were reacted with two equivalents of [Re2 (CO) 10]. In the case of bithiophene this produced, in good yield, the tetrametal biscarbene complex [Re2 (CO) 9C(OEt)C8H4S2C(OEt)Re2 (CO) 9] (8). In the thiophene and dimethylthieno[3,2–b]thiophene cases [Re2 (CO) 9C(OEt)C4H2SC(OEt)Re2 (CO) 9] (7) and [Re2 (CO) 9C(OEt)C8H6S2C(OEt)Re2 (CO) 9] (9) could be isolated in meagre quantities. This was ascribed to poor double lithiation (also steric hindrance in the case of 7). The carbene ligands reacted with water on the silica gel during column chromatography or in a control experiment with degassed water to produce aldehydes by reductive elimination from the metal. Protonation of the acylrhenate afforded rhenium hydrides which is also a potential precursor to aldehyde formation. This is believed to be a facile process for especially complex 9, isolated in very small quantity. Complexes 7–9 produced monocarbene aldehyde complexes [Re2(CO) 9C(OEt)C4H2SC(O)H] (12), [Re2 (CO) 9C(OEt)C8H4S2C(O)H] (13) and [Re2 (CO) 9C(OEt)C8H6S2C(O)H] (14), as well as dialdehyde compounds. Complexes 2 and 3 also produced aldehyde compounds. The formation of aldehydes from ethoxycarbene complexes is believed to involve hydroxycarbene intermediate species. Experiments were performed on [Re2 (CO) 10] and [Re(CO) 5Br]. They were reacted with 2–lithiumthienyl and then protonated. In the case of [Re2(CO) 10], hydride signals were observed on the 1H NMR spectrum, as well as aldehyde signals. In the case of [Re(CO) 5Br] there was strong NMR evidence indicating the formation of a hydroxycarbene complex. Complexes 1, 2, and 3 were reacted with Br2 (l). The metal–metal bonds were cleaved by the bromine to produce monorhenium carbene complexes [Re(CO) 4{C(OEt)C4H3S}Br] (4), [Re(CO) 4{C(OEt)C8H5S2}Br] (5), and [Re(CO) 4{C(OEt)C8H7S2}Br] 6) and [Re(CO) 5Br]. Complex 8 reacted with bromine to produce a monocleaved complex [Re2 (CO) 9C(OEt)C8H4S2C(OEt)Re(CO) 4Br] (11) and a biscleaved complex [Re(CO) 4Br{C(OEt)C8H4S2C(OEt)}Re2 (CO) 4Br] (10). Unique complexes [Re(CO) 4{C(OH)C4H3S}{μ–H}Re(CO) 4{C(O)C4H3S}] (15) and [Re(CO) 4{C(OH)C8H5S2}{μ–H}Re(CO) 4{C(O) C8H5S2}] (16) were obtained by starting with [Re(CO) 5Br] or [Re2 (CO) 10] and reacting them with 2–lithiumthienyl and 5–lithium–2,2'– bithienyl. These complexes were isolated from the column as very polar compounds after eluation of the aldehyde complexes. The dirhenium complex was obtained with a carbonyl– modified ligand (hydroxycarbene/acyl) on each of the metals. The complexes consist of two fragments held together by a hydrogen atom that bridges the two rhenium atoms (hydrido) and one that bridges the oxygen atoms of the carbene/acyl ligands (protonic). / Thesis (PhD)--University of Pretoria, 2011. / Chemistry / unrestricted

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The site of initial metal-carbonyl bond-breaking during ligand-exchange reactions in a series of octahedral metal carbonyls of the type (L2)M(CO)4 (M = Cr, Mo, W; L2 = diphos, phen, dipy) has been determined employing infrared spectroscopy and Fourier transform nuclear magnetic resonance spectroscopy. The results of this study reveal, for all metal carbonyl complexes of the type mentioned above, that loss of CO occurs exclusively at an axial position (cis to the bidentate ligand, I^)• The dynamic nature of the five-coordinate intermediates, such as (diphos)Mo(CO)3, (phen)M(CO)3 (M = Cr, Mo, W), and (dipy)Cr(CO)3, which are generated in solution upon CO dissociation, is reported and discussed. The results of this investigation confirm that these intermediates are fluxional on the time scale of CO-exchange process. A mechanism which describes the site of initial metal-carbonyl bond-breaking and the fluxionality of the five-coordinate intermediate during ligand-exchange reactions in the complexes (L2)M(CO)4 is proposed. A kinetic study of reactions of W(CO)6 with pseudo-halide anions (NCS-, NCO-, CN-) has been initiated. The results indicate that these reactions proceed via a bimolecular path, which involves initial attack of the pseudo-halide anion at a carbonyl carbon of W(CO)6,

This investigation applied the techniques of polarography and controlled potential electrolytic reduction to the study of a series of unsymmetrical benzils. Eleven unsymmetrical benzils were synthesized and studied in 50% ethanol-water (by volume) solvent systems buffered at pH 1, 5, and 13. The results showed that, with unsymmetrical benzils substituted with electron-withdrawing or electrondonating groups, the carbonyl closest to the ring with the least electron-donating power was the preferred reaction site. However, the dicarbonyl system was nevertheless reduced as a unit, and Hammett sigma values of disubstituted symmetrical benzils were approximately additive. This was ascertained by comparing the ease of reduction of eight unsymmetrical benzils to the ease of reduction of the corresponding disubstituted symmetrical benzils. It was proposed that the electrons were added to the oxygen of one carbonyl, and that the second carbonyl was involved in the reduction chiefly through complete polarization of the dicarbonyl system at the electrode. In general, the reduction of unsymmetrical benzils appeared to follow the same path previously reported for symmetrical benzils.

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